Stem cell factor ("SCF") is an early acting hematopoietic factor. See PCT WO 91/05795, entitled "Stem Cell Factor," herein incorporated by reference. SCF pharmaceutical compositions are known in the art, PCT WO 91/05795, supra, pages 20-21, however, an SCF pharmaceutical composition with increased shelf-life would be beneficial to both producers and consumers of the product.
In solution, SCF stability is limited by a number of degradation reactions, including cleavage, deamidation, and modification on RP-HPLC. Freeze drying (lyophilization) is considered useful and effective for preservation of many biologically active materials, including proteins. However, lyophilization induces its own stresses, including extreme concentration of the protein during the freezing process and removal of water, which may result in instability of the product. Hence, lyophilization may result in increased rates of crosslinking (covalent oligomer formation) and noncovalent aggregation, in addition to deamidation and oxidation, both of which can occur in the lyophilized state as well as the liquid state. Thus protective agents are often required to enhance stability of the drug by a number of mechanisms, including raising the glass transition of the formulation (that is, the temperature at which the composition changes from a fluid, rubbery and reactive state to a rigid, and, therefore less reactive state); acting as cryo- or lyoprotectants, (that is protective agents during the freezing and/or drying processes); and/or by replacing bound water molecules that are necessary for the conformational stability of the protein.
Amino acids have been noted in some cases to act as stabilizers for freeze-dried protein products. Sodium glutamate and lysine-HC1 have been reported to have cryoprotective effects on the freeze denaturation of a protein, lactate dehydrogenase (Seguro, et al., Cryobiology 27; 70-79 (1990)). Hora, et al. report the use of the L-arginine, L-carnitine chloride, or L-betaine as buffers for rhIL-2 (in Developments in Biological Standardization, Vol. 74, Karger, Basel, 1992). The formulations buffered with betaine showed dimer formation in accelerated testing, while arginine/carnitine-buffered formulations showed poor mechanical stability, leading to aggregation of the protein during production. Thus, the use of amino acids does not predictably enhance stability of lyophilized protein products.
Additionally, incorporation of small amounts of salt may, in some cases, destabilize lyophilized products, including proteins. The salts may be introduced during the pH adjustment of the formulation by addition of strong acids or bases. At the very low concentrations present in such a case, the salt may be trapped in the amorphous phase with the protein and may decrease the glass transition of the formulation or otherwise exert a detrimental effect on the stability of the protein drug. Elimination of such salt may therefore be desirable. Other classes of molecules, including mono- and di-saccharides, and polymers such as PVP have also been reported as stabilizers of lyophilized proteins, but, again, their utility is not predictable for any given protein product.